Steam reforming of a hydrocarbon to produce syngas is a well known process. One popular technique is to use an autothermal reformer in conjunction with a reforming exchanger. In such processes, one or more hydrocarbons and an oxygen source are supplied to the autothermal reformer. The combustion reaction is exothermic and supplies the heat needed for the catalytic reforming reaction that occurs in the autothermal reformer, which is endothermic, to produce a relatively hot reformed gas. The hot reformed gas from the autothermal reformer is then used as a heat source in the reforming exchanger, which is operated as an endothermic catalytic steam reforming zone. In the reforming exchanger, a feed of steam and hydrocarbon(s) is passed through open-ended tubes filled with reforming catalyst. The outlet ends of the tubes discharge the endothermically reformed gas near the shell side inlet where it mixes with the hot reformed gas from the autothermal reformer. The hot gas mixture is then passed through the shell countercurrently across or along the tubes in indirect heat exchange to supply the heat necessary for the endothermic reforming reaction to occur.
Reforming exchangers are in use commercially and are available, for example, from Kellogg, Brown, & Root LLC under the trade designation KRES. Various improvements to the reforming exchanger design have been made and are disclosed in, for example, U.S. Pat. Nos. 5,362,454; 6,855,272; 7,635,456; and 7,550,215.
These and other prior art exchangers, however, can be limited in their operation due to metal dusting concerns. Metal dusting occurs under certain temperature and pressure conditions when carbon monoxide (CO) in the gas mixture on the shell side of an exchanger corrodes metal surfaces of the exchanger (such as, for example, the tubes and walls of the exchanger), forming metal powder that interferes with efficient operation of the exchanger and results in thinning of the metal surfaces. To minimize metal dusting, the shell side outlet gas must be at a temperature sufficiently higher than the critical temperature for carbon formation, which is the chemical equilibrium temperature for the Boudouard reaction:2CO(g)←→CO2(g)+C(s) As a result, the extent of high grade waste heat recovery from the system is limited. Additionally, the mixed feed to the tube side of the exchanger can need to be pre-heated to address metal dusting concerns, which increases duty on an external heat source and limits heat integration opportunities.
There is a need, therefore, for improved systems and methods for reducing the potential for metal dusting within reforming exchangers, thereby widening the operating envelope of the reforming exchangers to include pressures and temperatures previously undesirable due to the risk of metal dusting.